Extension Communications
Extension 4-H Youth Building
Ames, Iowa 50011-3630
(515) 294-9915
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Extension Communications |
6/10/04
Contacts:
Mary Harris, Reiman Gardens, (515) 294-, maharris@iastate.edu
Jean McGuire,Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
For the week of June 7, 2004
Controlling Aphids in the Garden
By Mary Harris
Christina Reiman Butterfly Wing Curator
Reiman Gardens
Iowa State University
Most gardeners have seen them and few gardeners like them - aphids. Aphids are small insect pests that often are overlooked until an infestation has grown to be extensive. The plant damage aphids cause is not readily noticeable as they do not chew foliage or girdle stems. What aphids do is suck plant sap through tiny needle-like mouthparts. This week's Reiman's Pick is about biological (non-pesticide) control of aphids.
Aphids are small, seldom larger than 2 millimeters, soft-bodied insects most often found on the growing tips of plants. Depending upon the species, there are both winged and unwinged forms of aphids and the color may be green, red, purple, grey or black. Individuals are pear-shaped with long thin antennae and two "cornicles" on the hind-end resembling dual tailpipes. Aphids bear live young that closely resemble the adult aphid.
There are approximately 3,700 described aphid species, each of which is considered to be host specific, meaning each aphid species has one host plant species. Thus, each plant of agricultural or horticultural significance usually has at least one pest species of aphids.
The number of aphids in an infestation can increase rapidly due to the ability of aphids to reproduce without mating. An aphid infestation can become unsightly in two respects. First, the plant will develop withered, distorted tips and new leaves will be distorted or yellowed. Second, black sooty mold develops on the honeydew secretions the aphids produce.
Honeydew is often the first indication of an aphid infestation. The honeydew results from the aphid's unique filtration system that concentrates the scarce protein in the plant sap and excretes the excess sugar. These excretions are clear, sticky and sweet and provide the perfect medium on which the fungus called sooty mold will grow. Both honeydew and sooty mold can be rinsed or wiped off leaves. Also, ladybird beetles more commonly known as 'ladybugs,' will eat honeydew off leaves, after they eat the aphids producing the dew.
In addition to 'ladybugs,' there are other insects in the garden that provide biological control of aphids such as the larvae of green lacewings and hover flies, each of which are voracious consumers of aphids. The most effective natural enemies of aphids, however, are tiny parasitic wasps. These wasps are no larger than aphids and bear little resemblance to the larger wasps that can inflict painful stings upon us. These small wasps are parasitoids and when they 'sting' the aphid the result is far worse than pain for the unfortunate aphid.
Once the wasp deposits an egg into the aphid, a tiny larval parasite develops and proceeds to consume the aphid, from the inside out. Parasitized aphids, referred to as mummies, appear bloated and made of parchment. From each aphid mummy a new adult parasite will emerge that can parasitize hundreds more aphids.
Gardeners should give these wasps a chance to control aphids before resorting
to insecticide application. If an aphid infestation is developing quickly,
try spraying the infested tips with a strong stream of water, or even better;
spray them with a mild solution of dish soap. This will kill a few of the
aphids and dislodge most of them.
In all the glass houses at Reiman Gardens, these tactics are used along with
parasitic wasps to successfully control aphids. By using biological control
we are not exposing our guests or our butterflies to potentially harmful insecticides.
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Editors: Two color photos, suitable for publication, are available at right. Click on each thumbnail photo to go to the fullsized photo. The top picture's fullsize photo is 256K and the bottom picture's fullsize photo is 336K. Caption: Aphids are small, pear-shaped insects recognized by their "cornicles" which resemble dual tailpipes. |
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| Caption: Aphids often are attacked by parasitic wasps resulting in an aphid that is tan in color with an inflated appearance. |
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